Gas and oil reservoirs usually can be found in sedimentary rocks, which generally represent a set of high and low velocity contacting layers. Reservoir surveillance during production is a key to meeting goals of reduced operating costs and maximized recovery. Time-lapse seismic methods are well known method for monitoring changes in the reservoir during production. Seismic velocity and density changes in a producing reservoir depend on rock type, fluid properties, and the depletion mechanism. Time-lapse seismic responses may be caused by changes in reservoir saturation, pore fluid pressure changes during fluid injection or depletion, fractures, and temperature changes.
Enhanced oil recovery (EOR) is a general term used for increasing the amount of oil that can be extracted from a reservoir. EOR techniques include but are not limited to gas injection, thermal recovery (e.g. steam injection or steam flooding), and chemical injection. Areal field monitoring of EOR processes and other reservoir events has proven very successful as an aid to understanding the sometimes complex behavior of producing reservoirs. Seismic and other monitoring methods such as passive microseismic monitoring, satellite imagery and material balance calculations can all contribute to an integrated understanding of the reservoir changes.
A current method for providing a detailed picture of reservoir changes is surface seismic imaging, but there are difficulties associated with the method. An example of such a method is discussed in U.S. Pat. No. 6,717,867 which is hereby incorporated by reference. In surface seismic imaging methods, data quality can have enormous variations from field to field for various reasons including statics (which can vary from season to season) and multiples and reverberations which can dominate primary energy. Generally, stacking of high fold data is necessary to overcome these problems, but often even this stacking does not give sufficient signal-to-noise-ratio for EOR monitoring. Another difficulty with surface seismic monitoring is its high cost, especially on land. To monitor a land EOR operation that extends over approximately 50 square kilometres with a resolution of approximately 20 meters requires a huge investment in seismic operations. Ultimately, this huge expense can be attributed to the high fold required to achieve acceptable signal-to-noise levels.
Time lapse refraction seismology was first suggested as an alternative method for measuring changes in carbonate reservoirs. See Tatanova, Maria, Bakulin, Andrey, Kashtan, Boris, Korneev, Valeri, (2007), “Head-wave monitoring with virtual sources”, 77th Annual International Meeting, SEG, Expanded Abstracts, 2994-2998 (hereby incorporated by reference). According to this method, a seismic source is positioned somewhere above a reservoir (with higher compressional velocity than the surrounding rocks). The seismic source shoots into a geophone array and a crtitically refracted compressional (CRC) wave forms along the boundary of the reservoir and the overlying formation. The change in velocity of the head wave on the reservoir fluids and reservoir changes are easily detectable as time shifts in the seismic traces. One drawback of this method is that it requires a fast reservoir. Often the reservoir is a relatively slow rock surrounded by faster rocks and so this method cannot be used as it was originally conceived.
There is a need to develop a cost efficient method for monitoring a multi-layered system as it undergoes EOR operations and other reservoir changes.